It's ~. But copy&pasting this char every time is not something you want to do. You really should find out how to type it on your keyboard layout. From the Macnn forum:

Go to System Preferences>International>Input Menu and activate the Keyboard Viewer in the list of additional keyboards. Then click the flag menu in the right hand side of the menu bar to show the viewer. When you press alt (or option depending on what it is called on your keyboard) or alt-shift it will display what symbols will be input when you press that key combo.